1. Field of the Invention
The present invention teaches a fabrication method for thorium-uranium carbide and thorium-plutonium carbide fuels using the carbothermic reduction process. This method allows the preparation of kilogram quantities of fuel with good reproducibility of chemical and phase composition. Other methods employ liquid techniques that form carbide microspheres or alloying techniques that form alloys of thorium-uranium or thorium-plutonium. The liquid methods cannot process large quantities due to criticality concerns, while the alloy methods lack precise control of process conditions which leads to a poorly defined product. In particular, thorium-uranium carbide (Th,U)C and thorium-plutonium carbide (Th,Pu)C have been prepared by vacuum reduction of mixtures of the metallic oxides and carbon at elevated temperatures. Sintering at 1800.degree. C. and 2000.degree. C. produces fuel pellet densities of 87% and &gt;94% of theoretical, respectively, from the carbides.
Thorium-uranium carbide and thorium-plutonium carbide fuels are being considered as alternative candidates to uranium-plutonium carbide fuel for the fast breeder reactor. These fuels will minimize the use of plutonium and help serve as a deterrent to the proliferation of plutonium. One of the deterrents is the production of the .sub.92.sup.232 U isotope from .sub.90.sup.232 Th in the alternative fuel cycle. This isotope emits large amounts of gamma radiation and necessitates remote handling techniques. Another deterrent is the denaturing of the fissionable isotopes used to power the reactors. That is, .sub.92.sup.238 U is a non-spontaneous fissioning nuclide and will not sustain a chain reaction. Therefore, by mixing a certain amount of .sub.92.sup.238 U with .sub.92.sup.233 U or .sub.92.sup.235 U, denaturing is achieved, and complex and/or expensive isotope separation techniques would be necessary to obtain bomb grade materials.
The invention is a result of a contract with the Department of Energy.
2. Description of the Prior Art
Carbide fuels are potentially incompatible with the cladding of the fuel elements. Due to the limitations of current fuel synthesis techniques, the fuel composition cannot be controlled sufficiently well to yield a stable single-phase product. Too much carbon present, in the form of say the dicarbide, produces a carburization (embrittlement) of the cladding, while too little results in the free metal working its way through this material. A dry method for preparing the monocarbides PuC, UC, or (U,Pu)C with a metal-to-carbon ratio close to unity is described in U.S. Pat. No. 3,347,749. This patent teaches alloying U or Pu with chromium to alter the metal/carbon phase diagram allowing UC or PuC to form without requiring precise control of conditions. U.S. Pat. No. 3,813,344 teaches the preparation of (U,Pu)C by another dry technology which works for essentially the same reasons, and is similar to the carbothermic reduction of this invention. Note that the analogous plutonium fuel will not be readily synthesizable from the instant carbothermic reduction process without substantial oxygen impurity unless one is prepared to lose a large amount of plutonium metal by vaporization. Our invention, then, teaches a new method for preparing a fuel for use in nuclear reactors. Whereas the fuel itself is neither new nor impossible to prepare in another manner, the present method is safer and more economical.
3. Statement of the Objects
An object of the invention is to minimize the use of plutonium and reduce its theft and proliferation potential by providing an alternative to uraniun-plutonium carbide fuel for the fast breeder reactor.
Another object of the invention is the fabrication of alternative fuels with good reproducibility of chemical and phase composition.
Another object is to produce carbide fuels of the correct composition to avoid incompatibility with the cladding material of the fuel elements.
Another object is to produce carbide fuels in large quantities without criticality concerns during the manufacturing process as is currently a problem with liquid techniques.
Another object is to produce thorium-uranium carbide and thorium-plutonium carbide alternative fuel for the fast breeder reactor.
Another object is to produce thorium-uranium carbide and thorium-plutonium carbide by the carbothermic reduction process from a mixture of the appropriate metal oxides and pure carbon.
Other objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.